This invention relates to optical information media such as read-only optical disks and optical recording disks, a method for preparing the same, a method for performing recording or reading on the same, and a method for inspecting the same.
To record and store a vast quantity of information as typified by moving image information, advanced optical information media such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a recording/reading laser beam and increase the numerical aperture (NA) of a recording/reading optical system objective lens, thereby reducing the spot diameter of the recording/reading laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
For long-term recording of moving images of quality, an attempt was recently made to achieve a recording capacity of at least 4 folds of that of DVD, i.e., at least 20 GB/side, by reducing the recording/reading wavelength to about 400 nm and increasing the NA of the objective lens to about 0.85.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical recording medium relative to an optical system, which depends on the NA. The tilt margin is in proportion to
xcex/(txc2x7NA3)
wherein xcex denotes the wavelength of recording/reading beam and xe2x80x9ctxe2x80x9d denotes the thickness of a transparent substrate the recording/reading beam enters. If the optical recording medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8}xc2x7cos xcex8)xc2x7NA3/(n2xc2x7sin2xcex8)xe2x88x92
wherein n denotes the refractive index of the substrate and xcex8 is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness xe2x80x9ctxe2x80x9d of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a supporting substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the supporting substrate, and a light-transmitting layer of about 100 xcexcm thick is formed thereon as a thin substrate. Recording/reading beam reaches the pits or recording layer through the light-transmitting layer. This structure can achieve a higher recording density due to a higher NA because the substrate can be made extremely thin as compared with the prior art. One typical medium having such structure is disclosed in JP-A 10-289489. The medium is described therein as having a light-transmitting layer of photo-curable resin.
When the light-transmitting layer is formed of photo-curable resins such as UV-curable resins, however, the media can deflect due to shrinkage upon curing. Deflection can also occur when the media are stored in a hot humid environment. Once the media deflect, frequent errors can occur upon reading, and excessive deflection can cause the media to be unreadable.
JP-A 8-194968 describes an optical disk having a protective coat of resin. In this patent publication, use of the protective coat having a tensile elongation at break of at least 15% prevents the optical disk from deflection during storage in a hot humid environment. It is not described in this patent publication that a recording/reading beam is passed through the protective coat.
The inventors found that when a recording/reading beam is passed to the recording layer through a light-transmitting layer (or protective coat) of approximately 100 xcexcm thick, satisfactory recording/reading characteristics are not obtained merely by setting the tensile elongation at break of the light-transmitting layer at 15% or higher. Problems arise particularly when the diameter of a beam spot of a laser beam is reduced and recording and reading is performed at a high linear velocity. The most serious problem is that the focusing servo loses some stability. Another problem is an increased birefringence.
By reducing the recording/reading wavelength, increasing the NA of the objective lens to reduce the beam spot diameter, and increasing the linear velocity during recording and reading, there can be achieved a significant improvement in data transfer rate. Even a data transfer rate of 100 Mbps or higher is possible. With the start of the satellite digital broadcasting system at the end of 2000, image information of high quality is now delivered to home. A remarkable improvement in data transfer rate is thus demanded for recording such image information.
However, the focusing servo stability must be improved before the data transfer rate can be increased.
An object of the invention is to an optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer thereon wherein a recording or reading laser beam enters the recording layer through the light-transmitting layer, in which recording/reading characteristics are improved when the beam spot of a laser beam has a small diameter and the linear velocity is high.
The above and other objects are achieved by the present invention defined below.
(1) An optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer wherein a recording or reading laser beam enters the information recording layer through the light-transmitting layer,
said light-transmitting layer is formed of a resin and has a tensile strength at break of 5 to 40 MPa, a tensile elongation at break of 15 to 100%, and a tensile modulus of 40 to 1,000 MPa.
(2) The optical information medium of (1) wherein said light-transmitting layer has a thickness of 30 to 200 xcexcm.
(3) An optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer wherein a recording or reading laser beam enters the information recording layer through the light-transmitting layer,
said light-transmitting layer in an information recording region has a birefringence in absolute value of up to 20 nm at a wavelength of 630 nm and a birefringence distribution breadth of up to 20 nm at a wavelength of 630 nm.
(4) An optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer wherein a recording or reading laser beam enters the information recording layer through the light-transmitting layer,
said light-transmitting layer has a surface reflectivity of up to 10% at the wavelength of the recording or reading laser beam.
(5) An optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer wherein a recording or reading laser beam enters the information recording layer through the light-transmitting layer,
R/F is up to 10% wherein R is a residual error component of a focus error signal at a linear velocity during recording or reading and F is a peak-to-peak value of a focus sensitivity curve.
(6) An optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer wherein a recording or reading laser beam enters the information recording layer through the light-transmitting layer, wherein
said medium satisfies Wtxe2x89xa61840exe2x88x920.04V wherein said light-transmitting layer at its surface has a maximum waviness Wt (in nm) and said medium is moved at a linear velocity V (in m/s) during recording or reading, with the proviso that the recording or reading laser beam defines on the surface of said light-transmitting layer a beam spot having a diameter of up to 300 xcexcm,
(7) The optical information medium of (6) wherein said light-transmitting layer includes a light-transmitting sheet formed of a resin and an adhesive layer which joins the light-transmitting sheet to the supporting substrate side,
said adhesive layer comprising a cured product of a UV-curable resin and having an average thickness of 0.5 xcexcm to less than 5 xcexcm.
(8) The optical information medium of (6) or (7) wherein said light-transmitting layer includes a light-transmitting sheet formed of a resin and an adhesive layer which joins the light-transmitting sheet to the supporting substrate side,
said light-transmitting sheet being constructed from a polycarbonate, polyarylate or cyclic polyolefin by a casting technique.
(9) The optical information medium of any one of (3) to (8) which is to be operated at a linear velocity of at least 8 m/s.
(10) The optical information medium of any one of (3) to (9) on which recording or reading is performed by a system including an objective lens having a numerical aperture NA and emitting a recording or reading beam having a wavelength of xcex wherein xcex/NAxe2x89xa6780 nm.
(11) The optical information medium of any one of (3) to (10) which is the optical information medium of (1) or (2).
(12) A method for preparing the optical information medium of any one of (6) to (8), in which said light-transmitting layer includes a light-transmitting sheet formed of a resin and an adhesive layer which joins the light-transmitting sheet to the supporting substrate side, said adhesive layer being comprised of a cured product of a UV-curable resin,
said method comprising the step of irradiating UV radiation to a coating of the UV-curable resin for curing the resin to form said adhesive layer, the UV radiation irradiated having an energy density of up to 1,000 mW/cm2.
(13) In connection with an optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer, wherein said light-transmitting layer has a birefringence in absolute value of up to 20 nm at a wavelength of 630 nm and a birefringence distribution breadth of up to 20 nm at a wavelength of 630 nm,
a recording or reading method wherein recording or reading is performed by passing a recording or reading laser beam to said information recording layer through said light-transmitting layer.
(14) In connection with an optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer having a surface reflectivity of up to 10% at a recording or reading wavelength.
a recording or reading method wherein recording or reading is performed by passing a recording or reading laser beam to said information recording layer through said light-transmitting layer.
(15) A method for inspecting optical information media comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer wherein a recording or reading laser beam enters the information recording layer through the light-transmitting layer,
said method comprising selecting those optical information media in which R/F is up to 10% wherein R is a residual error component of a focus error signal at a linear velocity during recording or reading and F is a peak-to-peak value of a focus sensitivity curve.
In an optical information medium in which information is read by way of a light-transmitting layer of about 100 xcexcm thick, the present invention controls the tensile strength at break, tensile elongation at break and tensile modulus of the light-transmitting layer to specific ranges, respectively. The light-transmitting layer having specific physical properties has a reduced birefringence and a reduced birefringence distribution breadth, and serves to reduce the deflection and axial runout of the medium.
Because of the reduced deflection and reduced axial runout, especially because of the minimized axial runout, the medium undergoes a reduced axial runout acceleration when the linear velocity is increased. As a result, the residual error component (R) of a focus error signal at the increased linear velocity is 10% or less of the peak-to-peak value (F) of a focus sensitivity curve, whereby the focusing servo error at the increased transfer rate is reduced.
It is noted that no direct correlation exists between the deflection and the axial runout of the medium. A medium having a large deflection quantity tends to have a large axial runout quantity. However, a disk-shaped medium which has deflected like an umbrella, for example, experiences little increase of axial runout. On the other hand, a disk-shaped medium which has deflected while being twisted will undergo a large quantity of axial runout even when the deflection quantity measured is small.
Although it has not been proposed in the art to utilize R/F as the criterion for judging the focusing servo performance, the inventors have found that reducing R/F, specifically reducing R/F to 10% or below provides a great contribution to a reduction of jitter upon reading and to the prevention of writing errors. By measuring R/F at the linear velocity used for recording or reading (referred to as xe2x80x9coperating linear velocityxe2x80x9d), it can be judged whether or not recording and reading with high reliability is possible on the medium at the operating linear velocity. Therefore, the measurement of R/F can be utilized in the inspection of the medium. It is understood that the focusing servo does not fail even at R/F in excess of 10%.
In the medium of the invention, the utilization efficiency of light is increased on account of the reduced birefringence, which results in an increased read signal output.
It is understood that for a resin layer such as a light-transmitting layer, no direct correlation exists among tensile strength at break, tensile elongation at break and tensile modulus.